Integrated circuits are used in a wide variety of electronic equipment, including portable, or handheld, devices. Such handheld devices may include personal digital assistants (“PDA”), compact disc players, MPEG-1 Layer 3 digital audio (“MP3”) players, digital video disc players, AM/FM radio, a pager, cellular telephones, computer memory extension (commonly referred to as a thumb drive), etc.
To provide functionality, these handheld devices include one or more integrated circuits. For example, a thumb drive may include an integrated circuit for interfacing with a computer (for example, personal computer, laptop, server, workstation, etc.) via one of the ports of the computer (for example, Universal Serial Bus, parallel port, etc.) and at least one other memory integrated circuit (for example, flash memory). As such, when the thumb drive is coupled to a computer, data can be read from and written to the memory of the thumb drive. Accordingly, a user may store personalized information (for example, presentations, Internet access account information, etc.) on the thumb drive and use any computer to access the information.
Many of the integrated circuits used in handheld devices include mixed signal circuitry such as analog to digital converters (“ADC”) and digital to analog converters (“DAC”). As is known, analog to digital converters convert an analog signal into a corresponding digital value. There are different implementations of analog to digital converters having, accordingly, different resolution and sampling rate characteristics for intended ADC uses. For example, flash-type ADCs have a lower resolution (that is, less than 10 bits) and a fast conversion, or sample, rate that can typically achieve 1 Giga-samples-per-second. In contrast, integrating-type ADCs have a higher resolution (generally between 16-to-24 bits) and a slower conversion, or sample, rate of about 1 kilo-samples-per-second. Successive approximation-type ADCs come within the midrange of resolutions and sampling rates for analog-to-digital converters.
A successive approximation ADC converts an analog input to a digital output by successively comparing the analog input with digital bit values of finer resolution. Conventionally, a successive approximation ADC has a sample-and-hold circuit that receives an analog input signal. The sample-and-hold circuit output is provided to a comparator along with a capacitative digital-to-analog converter signal input provided in a feedback loop from a successive approximation controller. At the start of a conversion, the successive approximation ADC sets the output of a successive approximation register such that all bits except the most significant bit produces a logic low or “0”. The resulting output of the capacitative DAC is set to midcode of the analog-to-digital converter full-scale input. The comparator output is based on the difference between the capacitative DAC output and the sampled analog voltage.
While conventional successive approximation ADCs have been used in integrated circuits, limitations do exist. For instance, a conventional successive approximation ADC is prone to conversion error due inadequate settling in the sample-and-hold circuitry. Also, conventional successive approximation ADCs are subjected to increased cost and complexity in the fabrication process to incorporate those architectures.
Therefore, a need exists for an successive approximation analog-to-digital converter that that substantially overcomes the above mentioned limitations.